8.3.3.1 Requirements

In order to have these functions available, you must compile PHP
with support for the mysqli extension.

MySQL 8

When running a PHP version before 7.1.16, or PHP 7.2 before
7.2.4, set MySQL 8 Server's default password plugin to
mysql_native_password or else you will see
errors similar to The server requested authentication
method unknown to the client [caching_sha2_password]
even when caching_sha2_password is not
used.

This is because MySQL 8 defaults to caching_sha2_password, a
plugin that is not recognized by the older PHP (mysqlnd)
releases. Instead, change it by setting
default_authentication_plugin=mysql_native_password
in my.cnf. The
caching_sha2_password plugin will be
supported in a future PHP release. In the meantime, the
mysql_xdevapi
extension does support it.